The present invention relates generally to medium/high voltage electric cable connections. More particularly, the invention relates to a joint for medium/high voltage electric cables having a thermoplastic insulation system, and to a method for manufacturing the same.
Cables for transporting electric energy, particularly in the case of cables for medium or high voltage applications, include at least one cable core. The cable core is usually formed by an electrically conductive metal conductor sequentially covered by an insulation system. The insulation system is sequentially formed by an inner polymeric layer having semiconducting properties, an intermediate polymeric layer having electrically insulating properties, and an outer polymeric layer having semiconducting properties.
Cables for transporting electric energy at medium or high voltage generally include a screen layer surrounding the cable core, typically made of metal or of metal and polymeric semiconducting material. The screen layer can be made in form of wires (braids), of a tape helically wound around the cable core, or of a sheet longitudinally wrapped around the cable core.
The layers of the cable insulation system are commonly made from a polyolefin-based cross-linked polymer, in particular cross-linked polyethylene (XLPE), or elastomeric ethylene/propylene (EPR) or ethylene/propylene/diene (EPDM) copolymers, also cross-linked. The crosslinking step, carried out after extruding the polymeric material onto the conductor, gives the material satisfactory mechanical and electrical properties, even under high temperatures, both during conventional use and with current overload.
The crosslinking process of the polyolefin materials of the cable insulation system, particularly polyethylene (XLPE), requires addition to the polymeric material of a crosslinking agent, usually an organic peroxide, and subsequent heating at a certain temperature to cause peroxide cleavage and reaction. By-products are formed mainly from the decomposition of the organic peroxide, which may cause an accumulation of space charges and consequently electrical discharges and eventually insulation piercing, particularly in direct current (DC) energy cables. Therefore, the by-products of the crosslinking process must be removed by a long and cumbersome process of degassing, which is carried out, usually at a temperature of about 70° C.÷80° C., for a time from 15 days to about 2 months, depending on the cable dimensions, to cause migration and subsequent evaporation of the by-products from the cable core.
Energy cable accessories are used in an energy network to restore the insulation and electric field control over a cable portion where the conductor was exposed, such as in case of connection between two energy cables or between an energy cable and another network component, such as a transformer, a generator, a bare conductor of an overhead line or the like.
For medium/high voltage electric cables, joints can be built over the conductor connection by winding tapes of suitable materials in order to rebuild the insulating system of the cable, namely the inner semiconducting layer, the insulating layer and the outer semiconducting layer. As described by Thomas Worzyk, “Submarine Power Cables: Design, Installation, Repair, Environmental Aspects”, Chapter 4, Springer-Verlag Berlin Heidelberg 2009, this method is particularly suitable for joining high voltage cables for submarine installations, since it allows to obtain a joint (hereinafter referred to as “diameter joint”) which has a diameter slightly thicker than that of the joined cables, differently from the pre-moulded joints which inevitably have a diameter remarkably larger than the diameters of the joined cables. This allows an easier handling of the joined cables, especially when the joined cables are to be wound over a reel and then unwound for installation.
A proper and reliable rebuilding of the layers of the insulation system is essential to guarantee that the joint has the same performances, both thermo-mechanical and electrical, of the remaining portions of the cable insulation system. More in detail, the rebuilding of a diameter joint requires the use of tapes made with substantially the same materials of the cable insulation system, which are sequentially applied by winding them in a very accurate and clean way, to avoid formation of voids or other defects due to impurities which can give place to electric problems, such as partial discharge. Once the winding of the tape corresponding to the first inner layer of the cable insulation system is completed, the applied material is melted to become a continuous and homogeneous layer and then is cured under pressure to provide dimensional stability and to avoid deformation of the insulating system during deployment. The pressure applied during the curing step should be sufficient both to avoid formation of pores in the cured material, due to cross-linking by-product escaping, and to guarantee correct dimensions to the layer. This tape winding procedure is then repeated for the remaining two layers of the cable insulation system. The previous layer, i.e. the layer already wound around the cable, eludes deformation when the next layer is processed (by imparting a predefined pressure at a certain temperature to obtain crosslinking), because the previous layer is crosslinked.
For example, U.S. Pat. No. 4,204,896 discloses a method for producing an insulated joint between electrical conductors wherein heat is applied to the joined parts of the conductors covered by a cross-linked polyethylene material, that is in an uncured conditions and is closely confined within an enclosure. The enclosure is heated until the polyethylene material is cured, and pressure is applied to the joint in the enclosure while the joint is cooled, whereby the formation and/or persistence of voids in the cured joint is substantially suppressed.
In recent years, thermoplastic solutions have been developed in the field of medium and high voltage cables, in which the thermoplastic layers of the cable insulation system are based on polypropylene blends in admixture with an insulating fluid, as disclosed, for example in WO 02/27731 and WO 2004/066317. This solution avoids the crosslinking process.